Biermann Ch. Handbook of Pulping and Papermaking

Подождите немного. Документ загружается.

PULPING FUNDAMENTALS

A Vent

•fife

«ijf*'*^^

t"'*«'«•--'

(*•*- "'*.'«

*,«"

11 *

To Blow Tank

Fig. 3-11. A stationary batch digester with

indirect heatmg of the liquor. Reprinted from Fig. 3-12. A laboratory batch digester with

Making Pulp and Paper, ®1967 Crown product (below).

Zellerbach Corp., with permission.

GENERAL CHEMICAL PULPING

cess.

During the heating time, air and other

noncondensable gases from the digester are vent-

ed. 5) When the cook is completed, as deter-

mined by the kappa of pulp from the digester, the

contents of the digester are discharged to the blow

tank. 6) The digester is opened and the sequence

is repeated.

Continuous

digester

A continuous digester is a tube-shaped digest-

er where chips are moved through a course that

may contain elements of presteaming, liquor

impregnation, heating, cooking, and washing.

Chips enter and exit the digester continuously.

Continuous digesters tend to be more space effi-

cient, easier to control giving increased yields and

reduced chemical demand, labor-saving, and more

energy efficient than batch digesters.

Since continuous digesters are always pres-

surized, special feeders must be used to allow

chips at atmospheric pressure to enter the pres-

surized digester without allowing the contents of

the digester to be lost. Screw feeders are used for

materials like sawdust and straw. A moving plug

of fiber is used to make the seal. Rotary valves

(Fig. 3-16) work like revolving doors. A pocket

is filled with wood chips or other fiber source at

atmospheric pressure. When the valve is rotated

it is sealed from the atmosphere and then opened

into the digester where the contents are deposited.

Kamyr digesters (Figs. 3-13 to 3-15) are

large, vertical digesters, where chips enter the top

and exit the bottom continuously in a plug flow.

In 1992, 390 Kamyr digesters were operational

worldwide with a rated capacity of

million tons

per year. The chips are exposed to various areas

Fig. 3-13. The Kamyr continuoiis digester. Reprinted from Making Pulp and

Paper,

Zellerbach Corp., with permission.

i iii

o'~

I,-q

z::

A,,

oJf LMMI .......... lml]

~ ~

' I

I--

r,,)

8 r

o c

.c_

- ~'® ~

~~ Ii

~ E eh

• o J~l

GENERAL CHEMICAL PULPING

Fig. 3-15. A two-vessel Kamyr digester.

of the digester where they are impregnated with

liquor, heated to the cooking temperature, held at

the cooking temperature to effect pulping, and

partially washed. The pulp is then sent to the

blow tank.

In the typical process chips from the bin are

introduced to a low pressure pre-steaming chamber

where the chips are transported by a screw feed.

Here flash steam at atmospheric pressure preheats

the chips and drives off air so that liquor penetra-

tion will be enhanced. The chips pass through the

high pressure feeder where cooking liquor carries

them to the digester. Most of the liquor is re-

turned to the high pressure feeder. Impregnation

occurs for about 45 minutes at 130°C so cooking

will be much more uniform. Liquor heating

occurs in the zone formed by two external liquor

heaters. The cooking zone merely supplies the

appropriate retention time for cooking to take

place. The pulping reaction is terminated with the

addition of cold, wash liquor from the brown stock

washers that displaces the hot, cooking liquor.

The M&D digester (Messing and Durkee) is

a long digester inclined at a 45° angle. A diagram

of this is shown in Fig. 3-16, and an actual digest-

er is shown in Fig. 3-17. This digester is often

used for kraft pulping of sawdust or semi-chemical

pulping methods. It has some use for kraft pulp-

ing of wood chips. The chips or sawdust enter the

top,

go down one side of the digester, return back

up the other side being propelled by a conveyer

belt, and exit out the top. The cooking time is

about 30 minutes. A midfeather plate separates

the two sides of the digester. Since the size of

these shop-fabricated digesters are limited to about

2.4 m (8 ft) diameter, they are used for relatively

small production levels.

Another continuous digester is the Pandia

digester (Figs. 3-18 and 3-19), which is horizon-

tal,

uses a screw feed, and is often used in kraft

pulping of sawdust, semi-chemical pulping of

chips with short cooking times, and pulping of

nonwood fiber such as straw.

Digester

charge,

relief,

and blowing

The digester charge includes the wood chips

and cooking liquor in a digester. Digester relief

occurs while the chips are heated to temperature

and during the cooking process to relieve the

pressure caused by the formation of volatile gases.

These gases are released and condensed to im-

prove the pulping process. This removes air from

within and around the chips; increasing the liquor

circulation and evenness of cooking; reduces the

digester pressure, which reduces fiber damage

during the blow; and allows collection of turpen-

tine,

if desired.

Digester

blowing

occurs at the end of a cook

when the contents of a digester are cooled to about

100°C (212°F) and allowed to escape to atmo-

spheric pressure. There is usually sufficient force

in a full chemical pulp to cause fiber separation.

If the contents are blown from temperatures near

that of cooking, 170°C (340°F) for kraft cooking,

78 3. PULPING FUNDAMENTALS

Head

shaft

CROSS SECTION OF BAUER 18" x

20"

x 33"

INLET ROTARY VALVE, MODEL 71

End Bell

Relief

Primary

Exhaust

Entrance

Fig. 3-16. The M&D continuous digester. The inserts show various aspects

the rotary valve.

Courtesy

Andritz Sprout-Bauer, Inc.

GENERAL CHEMICAL PULPING

Fig. 3-17. The M & D continuous digester. To the right is the blow tank. The top insert shows

a hydrauUeally driven exit valve and the bottom insert shows an electric-motor driven exit valve.

the pulp often loses a significant (10-15%) amount

of its strength.

Blow tank

Blow tanks are large, cylindrical vessels that

receive the hot pulp from the digesters (Fig. 3-20).

Agitators mix the pulp from the digester with

dilute black liquor so the pulp slurry can be

pumped and metered for the correct consistency.

The heat of the hot gases from the blow tank are

recovered by the blow heat accumulator, a large

heat exchanger. Up to 1000 kg (2000 lb) of steam

per ton of pulp is generated by batch digesters and

some continuous digesters and must be condensed.

Continuous digesters like the Kamyr are able to

recover the heat more efficiently by liquor dis-

placement methods, although this technology is

being applied to batch digesters in the rapid

displacement heating method.

Condensation of the blow gases also decreas-

es pollution by recovering most of the volatile re-

duced sulfur compounds, organic compounds such

PULPING FUNDAMENTALS

Fig. 3-18. A Pandia continuous digester with two tubes. The insert shows the feed valve.

as methanol, and related materials known together

SLsfoul

condensate. The condensate is sometimes

used for dilution water in applications such as lime

mud dilution or brown stock washing where the

foul compounds will be trapped prior to a combus-

tion process. Non condensable gases are often

Discharger

Fig. 3-19. Pandia digester. Reprinted from

Making Pulp and Paper ^ ®1967 Crown

Zellerbach Corp., with permission.

Fig. 3-20. Blow tank. Reprinted from Making

Pulp

and Paper

®1967 Crown Zellerbach Corp.,

with permission.

GENERAL CHEMICAL PULPING

diverted to the lime kiln for combustion. The pulp

is then sent to be screened and cleaned in the

brown stock washers.

Liquor

Cooking or pulping liquors are aqueous

solutions of chemicals used for delignification of

wood by pulping.

Chemical concentration

The chemical concentration is measure of the

concentration of the pulping chemical in the

liquor. For example, in sulfite pulping the liquor

may be 6% SO2, indicating 6 grams of sulfite

chemical

(SO2

basis) per 100 ml of liquor. In this

case if the liquor:wood ratio is 4:1, the percent

chemical on wood is 24% as SO2. The following

is an important relationship, not the definition of

chemical concentration.

chemical concentration in liquor =

percent chemical on wood

liquor to wood ratio

Chemical charge (to a process), percent

chemical

on wood

(or

pulp for bleaching)

The chemical charge or chemical on wood is

another fundamental parameter of chemical pulp-

ing processes. It is the measure of the weight of

chemical used to process a material relative to the

weight of the material

itself.

This applies to any

chemical process, such as pulping and bleaching.

For example, typically, kraft pulping is carried out

with 25% total alkali on wood. This would

indicate 500 lb of alkali as sodium oxide (in the

U.S.

sodium-based chemicals are reported in terms

of sodium oxide) for 2000 lb of dry wood. Chem-

icals in sulfite pulping are expressed on an SOj

basis.

Also, when bleaching mechanical pulp, one

might use "0.5% sodium peroxide on pulp". This

means that for every one ton of oven-dry pulp, 10

lb of sodium peroxide are used.

, .11. mass dry chemical used ^^^^^

chemical charge= xl00%

mass dry material treated

Liquor to wood ratio

The liquor to wood ratio, liquor:wood, is

normally expressed as a ratio; typically, it has a

20 130

KAPPA NUMBER (LIGNIN CONTENT)

Fig. 3-21. Pulping selectivity curve.

value of 3:1 to 4:1 in full chemical pulping.

Rarely it is expressed as a percent. The numera-

tor may or may not include the weight of water

coming in with the chips, but either way it must

be specified to avoid ambiguity. The liquor to

wood ratio is kept as small as possible while

maintaining good digester operation, including

good liquor circulation for even cooking.

liquor ^ total pulping liquor mass

wood dry wood mass

Delignification selectivity

Delignification selectivity is an important

concept during pulping and bleaching operations

where it is desired to remove lignin while retain-

ing as much holocellulose as possible.

Delignification selectivity is the ratio of lignin

removal to carbohydrate removal during the

delignification

process.

While this ratio is seldom

measured directly, it is measured in a relative

manner by yield versus kappa plots as shown in

Fig. 3-21. Here condition "A" is more selective

than condition "B" since for a given kappa number

the yield is higher for "A" than for "B".

A high selectivity alone does not mean that

pulp "A" is better than "B" since the plot does

indicate the condition, such as the viscosity, of the

pulp.

For example, acid sulfite pulping is more

selective than kraft pulping; however, acid sulfite

PULPING FUNDAMENTALS

Inlet

Dilution = d

Reject = r

Fig. 3-22. Screening terminology in TIS 0605-

04,

Screening symbols, terminology, and equa-

tions.

pulp is weaker than kraft pulp because the cellu-

lose has a lower degree of polymerization due to

acid hydrolysis.

Hot stock refining, defiberating

Chemical pulps and, especially semi-chemical

pulps,

must be refined after the cooking process to

liberate the individual fibers. For chemical pulps,

this is relatively easily accomplished in the pres-

ence of hot liquor, which is the basis of hot stock

refining. Defiberating only separates the fibers for

thorough pulp washing; the fibers must be further

refined for papermaking.

Rejects, knotter or screener and pulp screener

Rejects are portions of wood, such as knots,

that do not sufficiently delignified to

fiber

ate. The

hard, poorly cooked lumps of wood are separated

Unscreened yield

Cooking time or H-factor

Fig. 3-23. Hypothetical relationship of total and

screened yields to cooking time or H-factor.

from pulp by coarse screening equipment (3/8 in.

holes) prior to pulp washing. Coarse sreenings

from sulfite pulping are not reused in bleached

grades of pulp. Coarse screening from kraft

pulping are repulped. Suggested screen terminol-

ogy is shown in Fig. 3-22. If pulps are under-

cooked, the screened yield may actually decrease

because much of the (potential) pulp is removed in

the screens, as shown in Fig. 3-23.

After washing, pulps are screened to remove

shives, dirt, and other contaminants to protect

processing equipment and the product's integrity.

Shorter fibers screen more easily, thus hardwoods

are easier to screen than softwoods. Redwood

pulps (long fibers) require about twice the screen

capacity as spruce pulps. Similarly, mechanical

pulps screen more easily than chemical pulps.

Recently, pulp is screened prior to washing for

improved washing efficiency. See Section 8.2 for

information on pressure screens.

Brown stock washers

Chemical pulps require more washing than

mechanical pulps in order to recover process

chemicals, but mechanical pulps require more

screening to remove shives, knots, and so forth.

If chemical pulps are not properly washed, foam-

ing will be a problem, additional make-up chemi-

cals will be needed, more bleaching chemicals will

be required, and additional pollutants will result.

There are several types of brown stock washers

available: 1. Diffuser

washing

systems, which are

slow, lead to high liquor dilution, and are there-

fore being phased out. 2. Press type washers. 3.

Countercurrent flow stock washers. 4. Rotary

vacuum drum

washer

with a drop length of pipe to

supply a vacuum with two to four stages (Plate

12).

5. Double wire press washers (Plates 14-

16).

6. Diffusion washers.

Rotary vacuum washers

Rotary vacuum drum washers are, by far, the

most common type used. These consist of a wire-

mesh covered cylinder that rotates in a tub of the

pulp slurry with valve arrangements to apply

vacuum as is suitable. As the drum contacts the

slurry, a vacuum is applied to thicken the stock.

The drum rotates past wash showers where the

pulp layer is washed with relatively clean water to

(more or less) displace black liquor. The vacuum

GENERAL CHEMICAL PULPING

is cut off beyond the wash showers and the pulp

mat is dislodged into a pulper. Most of the wash-

ing actually occurs at the pulpers between washing

stages. Daily loadings for brown stock washers

are about 8 t/w? (0.8 dry tons/ft^) for softwood, 6

t/m^ (0.6 dry tons/ft^) for hardwood, and 1-2 i/w?

(0.1-0.2 dry tons/ft^) for straw pulps. Hardwoods

are more difficult to wash than softwoods and

have higher chemical losses, partly due to the

higher ion-exchange capacity of hardwoods from

the carboxylic groups of their hemielluloses.

Typically, about 80% of the contaminants are

removed at each stage, so at least three stages are

required to thoroughly wash the pulp. Use of

additional stages results in lower water consump-

tion, higher recovery of chemicals, and lower pulp

dilution. Fig. 3-24 shows a typical configuration

of vacuum drum washers for pulp washing. Fig.

3-25 shows an individual vacuum drum washer.

Air is constantly sucked into the pulp sheet

and must be separated from the spent cooking

liquor to prevent foaming as it is fiirther pro-

cessed. This causes much foam to form in the

seal tanks. Chemical and mechanical defoamers

are often used here. The method of applying the

wash water influences foam formation.

Diffusion washing

Diffusion washing (Fig. 3-26) was first used

by Kamyr for washing chips at the bottom of their

digesters. The technique was extended for brown

stock washing and washing of pulp between

bleaching stages.

Double-wire

press

Plate 14 shows a double-wire press suitable

for market pulp production, brown stock washing,

pulp washing between bleaching stages, or stock

thickening. Plate 15 shows a double-wire press

designed for brown stock washing. Notice this

configuration incorporates the counter-current

washing technique.

Brown stock

The washed, screened pulp is called brown

stock. See Section 4.2 for more information on

pulp washing from a chemical recovery point of

view where dilution is considered.

Anthraquinone

(AQ),

soluble anthraquinone

(SAQ)

Anthraquinone (whose structure is shown in

Fig. 3-27) is a pulping additive used in kraft, soda

or alkaline sulfite processes to increase

delignification and decrease carbohydrate degrada-

tion. It works by going through a cycle which

leads to the reduction of lignin and the oxidation

of the reducing endgroup of cellulose from an

aldehyde to a carboxylic acid as shown in Fig. 3-

28.

In the latter case the carbohydrates are stabi-

lized against the alkaline peeling reaction, leading

to an increase in pulp yield. Because anthraqui-

none goes through a cyclic process, it is typically

used at about 0.1

on wood and results in a

1-3

increase in pulp yield. The use of anthraquinone

was first reported in 1977. Modifications of

anthraquinone, such as soluble anthraquinone

(SAQ,

Fig. 3-27), properly called 1,4-dihydro-

9,10-dihydroxyanthracene (DDA), have been

WASHER *\

WASHER n

« FILTRATE

STORAGE

Fig. 3-24. Brown stock washers showing countercurrent flow.